1. Field of the Invention
This invention relates to display processors including refresh memory storage and means for modifying the memory contents under computer control, and more particularly to such a system employing a resident processor which initiates a display segment, and performs display related tasks and also performs communication and editing functions between the generation of display segments.
2. Prior Art
Cathode ray tubes are commonly employed as output or display devices in computer systems. When the common type of low persistence cathode ray tube is employed the display must be repeated, or refreshed, at a relatively high rate to create the visual impression of a continuous display. While it would be technically possible to store the display contents in the computer's random access memory, and to use the computer to select character codes from the memory and to provide them to the display device, this high speed, highly repetitious task would require a large part of the computing capacity of a relatively high speed computer. Accordingly, special purpose "refresh" memories have typically been employed to store the character codes to be displayed on the CRT screen and hardwired circuits have been developed to perform the task of continuously generating the display from this memory, and performing certain auxilliary tasks such as "scrolling" a larger body of text than can be displayed at one instance to provide successive lines of this larger body of text for viewing. These refresh memories and associated hardware have been termed "display processors". They are typically associated with a computer which modifies the refresh memory contents from time to time as required.
The recent development of low cost, integrated circuit, digital numerical micro-processors, has resulted in the substitution of these processors for elaborate logic arrays in a wide variety of digital equipment. The micro-processors can often be programmed to perform a given set of digital functions at a lower cost than building a special purpose logic array of discrete components. The possibility of using a micro-processor as a substitute for the logic in a CRT display processor thus appears superficially attractive, but certain fundamental obstacles limit this substitution. Primarily, the speeds of micro-processors are not sufficient to allow them to perform the entire display generation task and it would be necessary to still provide some discrete digital hardware along with the micro-processor in a display processor system, using the micro-processor to replace only a portion of the processor's discrete logic. This trade-off is of only marginal economic advantage and accordingly micro-processors have been used with, but not as part of display processors.